This invention relates to a power supply for converting an alternating voltage supplied by an electric mains into a desired voltage, comprising rectifier means which include input terminals which are operative to receive the alternating voltage and output terminals which serve to supply a pulsating direct voltage, a first capacitor which is connected parallel to the output terminals, an inverter which includes input terminals which are connected to the first capacitor, and a current limiting circuit which is connected between one of the output terminals of the rectifier means and an electrode of the first capacitor in order to limit an inrush current occurring when the power supply is switched on, which current limiting circuit comprises a parallel connection of a first resistor and a first switching element, the first switching element being conceived to be in an electrically non-conductive state when the power supply is switched on and to change over to an electrically conductive state some time after switching on.
A power supply of this kind is known from U.S. Pat. No. 5,087,871. The first resistor serves to limit the current which after the switching on of the power supply, from the rectifier means to the first capacitor and the input terminals of the inverter. Because the first capacitor has not yet been charged upon switching on, this current could reach a very high value so that it might damage components of the power supply. Shortly after switching on, the first capacitor has been charged and the current is reduced to the much smaller value necessary to keep the inverter in operation. However, the value of this current is still so high that it would cause undesirable development of heat in the first resistor if it were to flow permanently through this resistor. Therefore, shortly after switching on the power supply the first resistor is bridged by the first switching element. The first switching element of the power supply known from the cited document is formed by a MOSFET which, after switching on, is turned on by a suitable control circuit. Other known switching elements, for example a bipolar transistor, a relay or a thyristor (see, for example DE-B-25 30 350) are also suitable for this purpose. All of said switching elements, however, have the drawback that they can become defective such that they are permanently in the conductive state. In that case they short-circuit the first resistor also during the switching on of the power supply, so that the inrush current is not limited. The user of the power supply notices this fact only after the inrush current has already caused damage, so that the entire power supply becomes defective.